Hepatitis C virus (HCV) infections represent a major worldwide health problem with an estimated 0.5-1.5% of the population infected. Approximately 70% of infected individuals develop chronic infections that may lead to cirrhosis and hepatocellular carcinoma. The introduction of diagnostic assays for antibodies to HCV has reduced the risk of transfusion associated HCV, but will have little impact on the spread of the disease by other routes. The development of efficacious antiviral treatments for this agent are essential to control the spread of the disease. A greater understanding of the replicative mechanisms of the virus will be essential in the development of antiviral therapies. A critical need exists for model systems to study replicative functions of the virus as well as tissue culture systems permissive for HCV replication. This proposal directly addresses these needs. The specific aims are 1) to construct full length clones of HCV and evaluate the clones for infectivity in a variety of cell culture systems; 2) to adapt HCV for replication in continuous cell lines by co- cultivation of cell lines with HCV-infected primary hepatocytes; and 3) to define the 3' HCV sequences required in cis for negative strand RNA replication. An additional aim is 4) to evaluate HCV-infected chimpanzees for the presence of antibody negative viral carriers and for the antibody response to native envelope proteins. The long term goals are to develop a better understanding at a molecular level of HCV replication. The model systems developed for these studies will be of great value in the evaluation of antiviral agents and will provide new targets for the development of antivirals. Accomplishments of the goals of this proposal will significantly impact the likelihood of developing efficacious treatments for this disease.